There are a number of techniques for examining moving webs which are commonly found in production processes in the paper industry, plastic industry, and other areas in which inspection across the entire width of the moving web to identify defects such as holes, wrinkles, streaks, and dirt are necessary with the web moving at speeds of up to approximately 5000 feet per minute (1,524 meters per minute). Thus the current technology employs such devices as slit scanners, multihead fixed field detectors, and flying spot scanners. The present invention is directed to the latter type of web defect analyzer and is particularly directed to the method of obtaining flying spot information and using that information to determine the characteristics of the moving web.
Representative of current flying spot web analyzing apparatus is U.S. Pat. No. 4,160,913, Brenholdt. This system, as shown in its FIG. 1, uses a source of illumination 17 (or line source 16 in combination with half-silver mirror 18) to illuminate a web 13 moving in a direction perpendicular to the scan path of flying spot 28. The flying spot is generated by oscillation of mirror 20 with the image of the flying spot passed through lens 19, a mask 24, filter 26, and photodetector 27. The amplitude of the signal is produced by the photodetector is thus a function of the reflected light, ambient light and associated electrical noise. All this information from the photodetector is amplified by pre-amplifier 32 (see FIG. 4 thereof) and differentiated by differentiator 36. A controlled gain amplifier 37 amplifies the output of the differentiator 36, the gain of the amplifier controlled by an automatic gain control (AGC) circuit 39. This output signal is then transferred to an active low pass filter 38, amplified again by amplifier 53 (see FIG. 5 thereof) and then fed to a characterization circuit 50 for determining the type of defect sensed across the moving web.
Differentiator 36 is used to emphasize the changes in the illumination information received from the moving web, in contrast to sensing the amplitude of the signal from the illuminated web. This is done apparently because it would be difficult to use an amplitude threshold as a means of detecting a flaw in the web in view of previously mentioned illumination variations which are not related to such defects. Rather, the differentiator creates a new signal whose amplitude is a function of the rate of change in the received light. The greater the instantaneous rate of increase in the signal, the higher the amplitude of the differentiated output. A rapid decrease in the illumination level of the signal concomittantly results in a very low instantaneously detected output from the differentiator. It is thus seen that the differentiated signal is not soley a function of the amplitude of the input signal but is also a function of its time variation. An abrupt change from a relatively high voltage to a low voltage (as generated by the photodetector 27) momentarily generates a negative value at the differentiated signal output. However, if the input signal stays low, the differentiated signal returns to its nominal value until another change is sensed.
The purpose of the AGC circuit is to set a base level for the rate of change based on the average rate of change across the web. This base level is established by charging a capacitor through an analog switch during the first part of each scan. Throughout the balance of the scan, the level established is subtracted from the differentiated signal, the resulting signal is then low pass filtered and transferred to the characterization circuit.
If the web has a hole, it is perceived by the Brenholdt disclosure as being darker than the surrounding web whereas a wrinkle is perceived as being a region of relatively greater brightness. As shown in FIG. 7 of Brenholdt, the signal from the detector shows holes as a slight positive anomaly and wrinkles as a negative anomaly. Waveform B of FIG. 7 shows that after differentiation, a small hole generates a positive pulse immediately followed by a negative going pulse whereas a wrinkle generates a negative pulse followed by a positive pulse.
The characterization circuit in Brenholdt seeks to distinguish such events (including determination of edges) and indicate their occurrence to the outside world. The system uses four signal events; namely, a large isolated negative pulse indicating the leading edge of the web; a smaller negative pulse immediately followed by a similarly sized positive pulse indicating a hole; a small positive pulse immediately followed by a similarly sized negative pulse indicating a wrinkle or streak; and a large isolated positive pulse indicating the trailing or far end of the web. The characterization circuit must therefore be able to detect such pulses and their time relationship with respect to each other.
In summary, the Brenholdt disclosure specifies a means by which a differentiated signal from a single sensor radiated by a flying spot scanning apparatus can be used to detect wrinkles, holes and edges associated with a moving web and to distinguish them by the order of the positive and negative transitions of the differentiated sensed signal. However, due to the fact that a differentiated signal is used, inherent noise problems associated with differentiation are present. Such problems include rapid changes in the overall illumination of the flying spot such as caused by changes in external illumination of the web in the area of the flying spot. Thus, for instance, if light from a skylight suddenly appeared on the web, this would generate a change in the illumination level. A change could also be caused by momentary blockage of an extraneous light source such as would result from a moving vehicle.
The present invention uses a different technique for producing a signal indicative of defects in a moving web by generating two flying spots adjacent to one another which scan the moving web. Instead of differentiating a single flying spot signal, the present invention produces an algebraically combined output signal from the two flying spots to provide web defect information. The present invention can alternatively be thought of as generating a single flying spot which is split into two adjacent halves. Regardless of how it is conceptually considered, the significance of the two simultaneous signals is that the system can simultaneously compare two adjacent points on the web to determine if the same amount of light is being respectively received. If the illumination is different for the two areas, then a defect of some sort (or edge) is being detected; the two flying spots in essence straddling the defect (or edge). A particular advantage of this arrangement is that abrupt changes in the level of ambient illumination do not affect the sensors differently. Thus an algebraic combination of the two signals is not affected.
In the present system comparison of the light reflected simultaneously from two adjacent flying spots on the web is sensed by spatially displaced photodetectors. The amplified signals from these photodetectors are combined in a particular manner to enable detection of relatively small differences in their respective illumination. Thus the present invention performs defect signal generation by operating in the space domain while the Brenholdt reference, by time differentiation of a single signal operates in the time domain.
Although U.S. Pat. No. 3,322,024, Preston, discloses an optical technique for the inspection of a transparent object for defects which uses two illumination sources, the apparatus operates in a manner completely unlike the present invention. In particular, the apparatus is for observing defects in a transparent medium such as glass, and it employs two light sources in order to develop illumination of alternate light and dark bands if a defect known as "wave" is present in the sheet glass. It does not use a flying spot technique nor two or more flying spot signals which are statially adjacent to each other with their signals algebraically combined to enable defect discrimination.
A number of other prior art references are directed light scanning apparatus such as set forth in Group I below
______________________________________ GROUP I Inventor U.S. Pat. No. Issue Date ______________________________________ Harries 2,993,403 1961 Greunke 3,060,319 1962 Poor 3,087,373 1963 Gaffard 3,338,130 1967 Dostal 3,532,408 1970 Brichard 3,543,033 1970 Corker 3,642,344 1972 ______________________________________
The references in Group I all describe optical scanning systems in which scanning is achieved by oscillating a mirror which, in each case, is supported by a torsion bar assembly. Although the present invention uses a scanning mirror supported by a taut band (which is similar to a torsion bar), none of these references disclose or suggest a scanning system having two flying spots adjacent to one another for scanning an output signal by the algebraic comparison of signals as received by separate photodetectors. They also neither disclose nor suggest adjusting the shape of the flying spot images in order to facilitate the detection of particular types of defects.
The references in Group II below show various types of scanning apparatus which, though relevant as showing the state of the art, neither disclose nor suggest the present invention including use of multiple flying spots for scanning the moving web and separate photodetectors each individually associated with one of the flying spots with the detection of defects obtained through comparison of the flying spot images rather than through use of a single flying spot image.
______________________________________ GROUP II Inventor U.S. Pat. No. Issue Date ______________________________________ Jorgensen 3,410,643 1968 McKown 3,516,743 1970 Kurotschka 3,589,792 1971 Bhullar et al 3,646,353 1972 West et al 3,686,008 1970 Rosin 3,687,025 1972 Kaneko et al 3,774,041 1973 Bertoya et al 3,779,649 1973 Shaw 3,871,773 1975 Buckson 3,970,857 1976 Reinke 4,003,626 1977 Craig 4,038,554 1977 ______________________________________
Another reference, U.S. Pat. No. 3,797,943, Naoao et al discloses a surface inspecting apparatus which incorporates a slit plate having a plurality of slits of different shapes which determine the field of vision for the object to be inspected. Although the apparatus uses a plurality of photoelectric conversion devices (see FIG. 1 thereof) each photodetector is associated with a different slit shaped in slit plate 7. It is used for discriminating particular types of defects since a defect which most closely fills a particular slit shape produces the greatest signal (see FIG. 6 thereof). This reference therefore does not disclose or suggest the use of a multiple flying spots with separate detection of each flying spot, nor of the comparison of these signals on an instantaneous basis in order to determine not only when a defect occurs but the type of defect through the relationship of the signals to one another.
Therefore, none of the prior art references cited are believed to disclose or suggest the present flying spot scanner system.